1. Field of the Invention
The present invention relates, in general, to an actuator provided by micro electro mechanical systems technology and optical actuator using the actuator and, more particularly, to an actuator and optical attenuator using the actuator, which utilizes an attractive force applied between comb-shaped electrodes.
2. Description of the Related Art
Generally, a comb-drive actuator refers to an actuator in which electrodes each having a plurality of fingers are interdigitated with each other and the positions of the electrodes can be changed using a voltage applied to the two electrodes.
Especially, a comb-drive actuator utilizing Micro Electro Mechanical Systems (MEMS) technology is excellent in the generation efficiency of an electrostatic force per unit area, the linearity of a drive displacement, etc. The comb-drive actuator has been frequently used in a gyro, an accelerometer, a mechanical filter, a variable optical attenuator, an optical switch, etc., since W. C. Tang, et al. developed a resonant microstructure (disclosed in U.S. Pat. No. 5,052,346) that is driven in parallel to the plane of a substrate. Further, A. P. Lee, et al. invented a comb-drive electrostatic actuator (disclosed in U.S. Pat. No. 5,969,848) that moves vertically to a substrate, and then proposed the alternative application of an actuator for corner cube reflectors. Further, B. Bhin, et al. invented an actuator (disclosed in U.S. Pat. No. 6,612,029) creating an out-of-plane rotatable flexure using comb-shaped electrodes.
When a micro shutter is inserted into an interval between two optical fibers aligned on a substrate using a comb-drive actuator driven in parallel to the plane of a surface, the quantity of light transmitted between the optical fibers can be controlled. Through the application of this principle, a variable optical attenuator or optical switch can be developed using the comb-drive actuator.
However, such a comb-drive scheme requires little current, but requires a high voltage to enhance a driving force. That is, in order to generate a constant displacement, a high voltage is required, so that a voltage amplification circuit must be separately constructed to control the high voltage using typical digital signals with a voltage of 5V or lower, thus increasing the sizes of elements and costs of products.
Alternatively, for a method of decreasing a drive voltage while maintaining the driving distance of a typical comb-drive actuator, there is a method of simply increasing the number of comb-shaped electrodes or decreasing the stiffness of the actuator.
FIGS. 1a to 1c are views showing a conventional comb-drive actuator and alternative actuators.
FIG. 1a illustrates a comb-drive actuator disclosed in U.S. Pat. No. 5,025,346 and proposed by W. C. Tang, et al., which includes a stationary comb electrode 1 fixed on a substrate (not shown) and a movable comb electrode 2 interdigitated with the stationary comb electrode 1. The movable comb electrode 2 is vertically connected to a horizontal axis 3. Both ends of the horizontal axis 3 are connected to vertical springs 4 and 5, respectively. In this case, the stationary comb electrode 1 and the vertical springs 4 and 5 are fixed on the substrate by anchors 6, 7 and 8, respectively. In such a structure, the conventional comb-drive actuator is characterized in that it moves in parallel to the substrate and performs translational motion.
As shown in FIG. 1a, it is assumed that, when a voltage V0 is applied to the comb-drive actuator having a spring constant k, a stationary displacement δ is generated. If a voltage required to generate the stationary displacement δ can be further decreased, a voltage amplification circuit can be removed and the size of the products using the actuator can be reduced, thus obtaining a plurality of advantages.
As described above, in the conventional comb-drive actuator, because a driving force is proportional to the square of a drive voltage, the driving force must be increased four times, or stiffness must be decreased by three quarters so as to decrease the drive voltage by half. Such alternative designs are shown in FIGS. 1b and 1c. 
FIG. 1b illustrates a design in which the stiffness is decreased by three quarters (0.25k) in the conventional comb-drive actuator. Further, FIG. 1c illustrates a design in which the number of comb-shaped electrodes is increased four times in the conventional comb-drive actuator, so that the driving force is increased four times.
In this case, if it is assumed that the resonant frequency of the structure of FIG. 1a is ω, the resonant frequency is decreased in such a way that the resonant frequency of that of FIG. 1b is 0.5ω), and the resonant frequency of that of FIG. 1c has a value close to 0.5ω. That is, it can be seen that the structures of FIGS. 1b and 1c are weak in external vibrations compared to the conventional structure of FIG. 1a. 
For example, it is assumed that there is a comb-drive actuator basically movable by 25 μm at a drive voltage of 20V. The resonant frequency of such an actuator can be designed to be approximately 1 KHz. If a comb-drive actuator that can be driven by a driving distance of about 25 μm at a drive voltage of 5V or lower is required to be developed by changing the number of comb-shaped electrodes and the stiffness, the mass of the actuator is increased and the stiffness is decreased in proportion to the increased number of comb-shaped electrodes, so that the resonant frequency of the structure is decreased to 200 Hz or lower.
Actually, when a comb-drive actuator is manufactured using the conventional structure and experiments are carried out with respect to the actuator, an applied voltage is about 20V if a driving distance is 25 μm, and a resonant frequency at this time is 900 Hz. If this drive voltage can be decreased to 5V or lower to be driven as a digital signal and the driving distance can be maintained at 25 μm, a drive voltage amplification circuit can be removed in the applications, such as the above-described variable optical attenuator, and the size of products using the actuator is decreased, thus strengthening the competitiveness of the products. However, when the comb-drive actuator is designed and manufactured using the conventional structure, the design of products using the actuator is limited in that, since experimental results show that a resonant frequency is lower than 250 Hz (the number of comb-shaped electrodes is increased eight times and stiffness is decreased by half), response variations are generated depending on external vibrations and a response drift occurs due to the weight of the structure itself.
Moreover, if the above-described conventional comb-drive actuator is used for an optical attenuator that controls the power of an optical signal in an optical network, a method of increasing a driving distance relative to a voltage by simply increasing the number of the comb-shaped electrodes or by decreasing the stiffness of springs, is problematic in that the resonant frequency of the structure is decreased and varied sensitively to external vibrations. That is, the optical attenuator must attenuate incident light to a certain level, but there occur several problems including a problem that the attenuator cannot satisfy the fundamental functions thereof when external vibrations are applied to the optical attenuator.